Solar collection assemblies, sometimes referred to as parabolic trough receivers, are used to collect radiation from the sun for conversion to a usable form of energy. Solar collection assemblies include mirrored surfaces, supported by truss-like structures, configured to track the sun's movement across the sky and collect solar radiation. The mirrored surfaces reflect the sun's rays toward a heat collection element (HCE), commonly referred to as a receiver, maintained at a focal point of the reflector's parabolic shape. A heat transfer fluid flows through the receiver and is heated by the reflected and concentrated radiation. The fluid is ultimately delivered to a heat transfer system where the heat is converted to electricity or other usable form of energy. A typical solar array field may use up to 80,000 or more receivers.
The primary function of the heat collection element is to absorb as much of the incident solar energy as possible, and to re-emit as little of that energy as possible in the form of thermal radiation. This goal is achieved by enclosing an inner absorber tube within a surrounding outer glass envelope. The outer surface of the absorber tube is coated with a solar selective coating which is designed to absorb as much of the incident solar energy as possible (typically around 95% or so) while minimizing the re-radiated losses due to thermal radiation. The outer glass envelope plays a critical role in the heat collection element. The glass envelope allows the formation of a vacuum in the annular space between the inner surface of the glass envelope and the outer surface of the absorber tube. This vacuum prevents energy loss from the absorber tube by preventing heat conduction from the absorber tube to the environment. This, in turn, maximizes the amount of absorbed solar energy which enters the heat transfer fluid within the absorber for later conversion to a more useable form.
In addition to the outer glass envelope, a leak-free sealing mechanism must be provided to enable and maintain the vacuum between the glass envelope and the absorber tube. The implementation of this seal is made difficult by the fact that the absorber tube and the glass envelope, being made of different materials, have different thermal expansion rates, and thus, expand by different amounts when heated.
A bellows interface interconnects the transfer absorber tube and the glass envelope. The purpose of the bellows is to allow for the different thermal growth rates between the glass envelope and the steel absorber tube without placing undue stress on the glass, while at the same time maintaining the vacuum conditions within an annulus between the tube and the envelope. In compensating for the expansion difference, typical bellows used in conventional receivers shield a portion of the absorber tube. This is due to the fact that the typical bellows consist of a series of convolutions which run axially along the absorber tube and thus form a barrier between the rays of the sun and the surface of the absorber tube beneath the bellows. This shielding is a significant drawback of the current bellows design. The extended length of the bellows is needed to enable proper operation of the bellows, but the extended shielding reduces the effective length of the radiation exposure to the absorber tube. The reduction in effective length in turn reduces the efficiency of the receiver, which reduces the efficiency of the solar collection assembly. The bellows on typical solar receivers shade approximately 3% to 5% of the available absorber tube surface. This results in a corresponding percentage decrease in the receiver's operating efficiency. Another constraint with the current bellows designs is their need to withstand repeated expansions and contractions, typically more than 10000 cycles over a receiver's lifetime. If the glass-metal seal provided by the bellows is broken, the ability of the receiver to perform its intended function is diminished.
Therefore, there is a need in the art to reduce the size of the bellows so as to minimize shading while maintaining a sealed connection between the metal tube and the glass envelope. And there is a need to simplify the design of the bellows so as to further improve the assembly of the receiver.